The present invention relates to the art of parameter measurement, and more particularly to a method and apparatus for correcting a parameter measurement for stray ground voltages in the equipment to which the measurement apparatus is attached.
Physical parameters such as position, temperature, pressure, liquid level, etc. are conventionally measured by systems including two principal elements--a parameter sensor or transducer for converting the physical parameter into an electrical parameter (such as resistance, capacitance, or inductance), and an electrical circuit for measuring the electrical parameter. Most often, it is the electrical resistance of the sensor which varies with the parameter being measured. In automotive and agricultural equipment, for example, fuel level senders, oil pressure sensors, water temperature sensors, etc. all are generally variable resistance elements whose resistance changes in direct relation to the parameter being measured. The resistance of the sensor element is determined by applying a known current through the sensor, and then measuring the resulting voltage drop across the sensor. As long as the applied current is maintained constant, the voltage drop across the sensor is directly proportional to the resistance of the sensor element.
The sensor must be mounted at the place on the equipment at which the parameter is to be measured. Often the sensor must be mounted an appreciable distance from the circuitry which actually generates the sensor current and measures the resulting sensor voltage. Although separate leads could be connected between each end of the sensor and a corresponding terminal of the measurement circuitry, in automotive applications it is not uncommon instead to connect one terminal of the sensor directly to the metal structure of the vehicle. The metal structure is usually grounded, and hence provides a ground return path for current between the sensor and the measurement circuitry. Thus, only one additional lead must be provided, connected between the other terminal on the sensor and the measurement circuitry.
When one end of the sensor is directly connected to a metal structure, it is implicitly assumed that the ground potential at that point on the structure is the same as the ground potential at the point on the structure to which the measurement circuitry is attached. This may not always be the case, however. There may be high circulating currents in the metal structure due to other, high current draw devices which also use the structure as a ground return path. The existence of the high circulating currents introduces measurable voltage drops between various points on the structure. Consequently, it is possible that the "ground" voltage at the point of connection of the sensor element to the structure may be shifted above or below ground voltage at the point of connection of the measurement circuitry to the structure. The voltage measured by the measurement circuitry represents the additive sum of the voltage across the sensor and the ground shift voltage. The parameter reading generated by the measurement circuitry will therefore be incorrect by an amount related to the magnitude of the ground shift potential.